Desorption of adsorbed water and other impurities from oxides can be accomplished by conventional, resistively heated thermal cells. A detailed explanation of a conventional thermal cell is taught by S. H. Moon et al., A SIMPLE-DESIGN HIGH VACUUM INFRARED CELL FOR IN-SITU STUDIES OF SUPPORTED METAL CATALYSTS, Ind. Eng. Chem. Fundam., 20, 396-399 (1981), hereby incorporated by reference. The article reports a vacuum IR cell. The cell uses commercial vacuum adaptors containing O-ring seals, that degrade at high temperature, as the IR window holders. The adaptors may be disassembled easily for sample introduction into the cell. The device is cooled to prevent degradation of the seals. The cell was operated continuously and periodically at 550.degree. C. without use of a beam of electromagnetic radiation. A basic design for the cell is presented.
The IR cell consists of several main parts. The center part is a quartz tube of 1 in. O.D. and 7/8 in. I.D. and has a 1/4 in. quartz tube and a high vacuum stopcock near each end to permit gas introduction or flow. A sample holder is positioned in the center of the tube so that a catalyst wafer is located at the center of the cell. The center unit is wrapped with heating wire and insulation for high temperature operation of the cell and sample. Temperature is measured by a thermocouple positioned directly behind the sample holder. The holder is made of quartz tube having an O.D. about 7/8 in. Two annular quartz plates with 3/8 in. center holes are attached to one end of the sample holder. The plates are spaced to position the catalyst wafer between them perpendicular to the IR beam. The ends of the cell are sealed by IR windows constructed of NaCl.
Activation and cleaning of oxide surfaces using electromagnetic radiation has been taught by A. D. Abbate et al., ACTIVATION AND CLEANING OF OXIDE SURFACES BY A CW CO.sub.2 LASER, Surface Science 136, L19-L24 (1984), hereby incorporated by reference. This article teaches that electromagnetic radiation heating is a generally useful tool for the cleaning and activation of portions of oxide surfaces. Such surfaces can be used as catalysts. The oxide surface itself is heated. The technique is particularly suitable for samples which have a small thermal conductivity. According to the article, oxide samples were pressed into two centimeter diameter disks which were sandwiched between copper washers with an 11 millimeter aperture. A stainless steel flange pressed the sandwich against the recessed ledge in a copper block. It was resistively heated to 250.degree. C. or cooled with liquid nitrogen. The assembly was enclosed in a vacuum stainless steel cell. According to the article, laser heating has been shown to accelerate the dehydroxylation of silica disks. The article reports physical removal of adsorbed water and hydrogen bonded hydroxyls on silica, Al.sub.2 O.sub.3 and zinc oxide surfaces. These are not zeolites which have distinct three dimentional crystalline structures. The reference indicated that the radical intensity distribution was not Gaussian, that is, not TEM.sub.oo. Samples were prepared by laser heating alone or with simultaneous heating of the sample holder. Notwithstanding this, all the samples treated in this reference were prepared by resistively heating the cell to 140.degree. C., in addition to laser heating. Irradiation times were 10 to 30 min. The reference further indicated that heating uniformity would be improved with better control of the laser modes.
The prior art teaches electromagnetic irradiation utilizing awkward procedures and apparatus. The prior devices necessitate heating the cell as well as the sample. The devices require substantial disassembly for sample manipulation. Applicants have overcome disadvantages through their inventive process and apparatus for selectively activating the entire sample by laser heating just the sample for a very brief time period.